Certain aspects of this invention relate to staged, flat-flame combustion within a furnace which contains molten material such as glass to be affected by heat generated by the combustion. The combustion is also used to affect the oxidation state of the iron in the molten glass in the refining or fining section of the glass melting furnace.
In many industrial heating processes fired with fuel and oxidant, products of fuel combustion in the melting end of the furnace may interact or react with molten material and often cause undesirable effects. For example, a fuel rich flame impinging over melting glass in a glass melting furnace is known to cause color change in glass product due to redox change of the glass melt exposed to the fuel rich flame.
Nitrogen oxides (NOx) can be a pollutant generated during combustion and it is desirable to reduce their generation in carrying out combustion. It is known that combustion may be carried out with reduced NOx generation by using technically pure oxygen or oxygen-enriched air as the oxidant as this reduces the amount of nitrogen provided to the combustion reaction on an equivalent oxygen basis. However, the use of an oxidant having a higher oxygen concentration than that of air causes the combustion reaction to run at a higher temperature and this higher temperature kinetically favors the formation of NOx. Staged combustion has been used to reduce NOx generation. U.S. Pat. Nos. 5,611,682, 6,524,097 and U.S. Pat. No. 7,390,189 (hereby incorporated by reference) describe a staged oxy-fuel burner for producing a generally flat fuel rich flame overlying a highly radiative fuel lean flame.
Glass that is fairly neutral or clear in color, and highly transparent to visible light (e.g., at least 75% transmissive, or even more preferably at least 80% transmissive), is desirable for supply of the solar glass industry, among other industries. One way of achieving such a glass is to use very pure base glass materials (e.g., substantially free of colorants such as iron). However, base materials with a high degree of purity are expensive and thus not always desirable and/or convenient. In other words, for example, the removal of iron from glass raw materials has certain practical and/or economical limits. Other methods to produce “clear” glass is to introduce exotic batch materials that help with oxidization.
Control of the oxidative state of the glass is useful in producing “clear” or brilliant glass. In some cases, it is desirable to have the iron in a very low FeO (ferrous state) content (e.g., as discussed in US20060581784; hereby incorporated by reference). This can be advantageous since ferrous iron (Fe 2+; FeO) is a much stronger colorant than is ferric iron (Fe 3+; Fe2O3).
There is a need in this art for a furnace and method for controlling the oxidative state of transition metals in a molten material.